1. Field of the Invention
The present invention relates to a semiconductor device having a chip-on-chip structure having semiconductor chips stacked, on their surface, with other semiconductor chips.
2. Description of Related Art
There is a multi-chip module that is one form of semiconductor devices intended to improve the transmission speed of the signals at between devices. In the multi-chip module, a plurality of semiconductor chips are mounted on a circuit board in one package. By shortening the wiring connecting between the semiconductor chips, high speed transmission of a signal is achieved. A plurality of semiconductor chips forming functional elements are mounted on the circuit board, wherein each semiconductor chip is connected face down on the circuit board (flip-chip connection). The circuit board, usually, uses an insulating board formed with multiple levels of wiring thereon. Namely, the circuit board includes the wiring on a surface layer, the wiring on inner levels, and the wiring of inter-level connection.
The semiconductor chip mounted on the circuit board in a certain case is further stacked with other semiconductor chips thereby forming a chip-on-chip structure.
In the multi-chip module thus structured, by shortening the length of the wiring connecting between the circuit board and the semiconductor chip and between the semiconductor chips in the circuit board, it is hoped that high speed transmission of signals between the functional elements formed on the semiconductor chips will be realized. Also, because connection is possible to another circuit board through the bumps provided at the underside of the circuit board, the wiring length for external connection is comparatively short thus making it possible to externally transmit a signal with a certain degree of speed.
However, in the multi-chip module, the wirings on the circuit board are greater in width and pitch as compared to the wiring made by the semiconductor process.
Consequently, in order to mount semiconductor chips with high density on the circuit board, there is a need to increase the number of levels of wirings in order for preventing against interference between the wirings, resulting in an increased wiring length. This makes it impossible to sufficiently increase the transmission speed of signals.
Meanwhile, the semiconductor chips are externally connected through the circuit board using an insulating substrate and hence the possible wiring-length reduction is limited. This makes it impossible to sufficiently increase the signal transmission speed over the external connection.